The Size, Structure, and Variability of Late-Type Stars Measured ...

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1.4 Stellar Evolution and AGB Stars
Late-type AGB stars are ideally suited for observation with the ISI. These stars
are nearing the end of their lives, and have grown to several hundred times their original
size. Extremely luminous, they have relatively cool temperatures on the order of 3000 K.
Because of their mostly infrared flux and their relatively large angular size, the ISI is able
to obtain accurate visibilities on a number of these stars.
The following section discusses the stellar life cycle and, in particular, the workings
of AGB stars like the ones considered within this thesis. It is included to serve as a background
for those unfamiliar with stellar structure and evolution and to provide a framework
of how AGB stars, including the Mira variables and supergiants discussed within this thesis,
fit into stellar populations. Most of the information pertaining to stellar evolution comes
from Prialnik (2000) [78] and Padmanabhan (2001) [74].
A star, by definition, is a gravitationally bound object radiating due to an internal
source. Typically, stars are spherical because gravity acts in a radially symmetric way, and
their energy source is nuclear fusion occuring in the core. However, energy released from
gravitational self-attraction or stored in the heat capacity of its matter is not necessarily
negligible for stellar phenomenon which occur over time scales characteristic of these processes.
Although elemental composition does vary between stars, the total stellar mass is
the defining parameter for a star. It determines the size, structure, lifetime, and evolutionary
course a star will have. The Hertzsprung-Russell (H-R) diagram is a plot of the surface
temperature vs. luminosity for a collection of stars. Individual stars are points in the plane
and variations occur in two dimensions due to the one-parameter variations in both mass
and age. Patterns do exist in the H-R diagram, and the populations and evolutionary tracks
of stars within it can be reproduced quite well from theoretical predictions. Figure 1.5 shows
an H-R diagram of almost 9000 stars from Perryman et al. (1995) [77]. The ordinate of the
plot is B-V magnitude instead of temperature although these quantities are directly related
for the most part. Only stars whose parallax was measured with precision better than 10%
were plotted. Hence, the collection is not necessarily a representative pool of all stars. A
long band of stars known as the main sequence is seen to contain most stars. These stars
are in the longest portion of their life-cycle and vary in mass from about 0.1 M ⊙ at the
bottom-right corner, to 10 M ⊙ in the upper-left. Another grouping of stars occurs at higher
luminosities and cooler temperatures in the top-right portion of the diagram. These are